In certain regions aircraft and ground stations communicate over VHF radio using what is commonly known as a “multi-carrier technique.” That is, multiple ground transmitters, each transmitting the same voice signal over a separate carrier, are positioned throughout a geographic area. Typically the transmitters are separated by a distance of 50 to 100 miles and transmit on carrier frequencies which are individually offset from the channel's nominal frequency by offsets such as +/−5 kHz or +/−7.5 kHz. Additional information regarding the details of VHF multi-carrier communication can be found in ICAO Annex 10 Volume III, dated July 1995 with Amendment 71, dated 7 Nov. 1996 (herein incorporated in its entirety by reference).
So that a pilot is not continuously exposed to static noise in the absence of active transmissions, the aircraft's VHF radio implements a noise-based squelch. Within the intermediate frequency (IF) section of the radio, the noise-based squelch listens for noise in a frequency band slightly offset from the nominal frequency (i.e., the information bandwidth) of the selected channel. The frequency band monitored for noise normally does not contain transmitted intelligence. In radios which incorporate a detector this same technique described for the IF can be applied after the detector by monitoring noise at audio frequency. When not receiving a signal within the channel, IF noise is generated within the radio that can be observed within this band. When the noise-based squelch observes this noise and determines that the noise is above a pre-define level, the squelch is applied to suppress the output of audio to the pilot. When a VHF radio signal is received, the noise level within the observed band is reduced. This reduction of the IF noise indicates reception of a radio signal which, if it is reduced below a predetermined level, triggers the noise-based squelch to allow audio output to the pilot.
A problem occurs, however, when using the noise-based squelch in multi-carrier regions. Reception of multiple VHF radio voice signals within the radio's pass band will cause beat-frequency tones to appear within the frequency band observed by the VHF radio's noise-based squelch. Under these conditions, the VHF radio's noise-based squelch will “see” the beat-frequency tones and misinterpret them as a high level of noise and continue to apply the squelch to the audio output. The beat frequency's appearance in place of the noise effectively “locks” the squelch for the duration of the multi-carrier transmission, preventing the pilot from receiving voice information from the VHF radio. One technique for mitigating this effect is to fall back to automatic gain control (AGC) based squelch in multi-carrier regions. However, an AGC-based squelch provides less inherent ability than the noise-based squelch to distinguish the presence of a valid signal from noise. If the AGC-based squelch is adjusted to provide the same sensitivity as the noise-based squelch then there is a possibility that the AGC-based squelch will provide false squelch openings, which cause static noise to reach the pilot and contributes to pilot distraction. To prevent this, the AGC-based squelch is typically set to a much lower sensitivity than the noise-based squelch.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification there is a need in the art for improved methods and systems for signal analysis in multiple radio carrier environments.